Recording media which employ an electron-donating achromatic dye and an electron-accepting compound include pressure sensitive paper, thermosensitive paper, light- and pressure-sensitive paper, electric heat-sensitive paper, thermosensitive transfer paper, and the like, and are disclosed in detail, for example, in British Patent 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, JP-B-60-23992, (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-57-179836, JP-A-60-123556, and JP-A-60-123557 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Such prior art recording media in which an electron-donating leuco dye and an electron-accepting compound are used have a disadvantage in that a non-image portions inadvertently develop color caused by heating and the like after image recording. Consequently, intensive studies have been and continue to be conducted in the field with the aim of solving this problem.
As an attempt to improve stability of recording media, a recording medium in which an electron-donating achromatic dye is encapsulated in microcapsules has been proposed to insulate an electron-accepting compound from the achromatic dye as disclosed in JP-A-2-81671. However, satisfactory shelf life of images cannot be obtained even by the use of this method.
As a method for the development of image by uniform heating of a recording medium after image exposure, a process has been proposed in which a photo-polymerizable composition and heat-sensitive coloring medium are used and a visible image is formed by heat developing treatment. According to this process, the photo-polymerizable composition is encapsulated in microcapsules, and as a result, the cured portions of the photo-polymerizable composition after image exposure are insulated from contact with the heat-sensitive coloring material so as to prevent color development in the cured portions. By this process, however, development of color on the photo-cured portions cannot be prevented satisfactorily at the time of the heat developing treatment, and, consequently, development of fog occurs on non-image portions.
In addition, another method has been proposed which comprises using microcapsules containing an electron-donating achromatic dye and a photo-polymerizable composition as disclosed in JP-B-60-30931 and a composition of an electron-accepting compound and a photo-polymerizable composition or a composition of a photo-polymerizable electron-accepting compound. This process, however, is not satisfactory with regard to the color developing density.